They want you to show your work because you are more likely to understand how to do the problem by showing your work, and are more likely to catch an error. Generally, if you show your work, you may get partial credit for a problem with a wrong answer if the instructor can see that you know what you’re doing and can identify where you went wrong.

Show your work, use graphs and diagrams when appropriate. They just want you to show everything you did to got the answer and if drawing a picture (geometry) or a graph (algebra/calculus) would be applicable then it can supplement your answer.